Pal identification circuit

Abstract

In a PAL color television signal receiver including a color demodulator, a source of first timing signals for determining the line by line switching of the color demodulator, and a source of second timing signals coincident with the burst interval of the color signal, a circuit for identifying an incorrect mode of line by line color demodulator switching. A signal representative of the periodic, alternating phase burst component of the color signal is sampled every other image line to derive an identification signal representative of the timing of the burst component. The identification signal is utilized to alter the operation of the source of timing signals to produce timing signals corresponding to a correct switching mode. A sampling circuit employed to derive the identification signal is keyed in response to the first and second timing signals such that the derived signal is suitable for use as a switching mode identification signal, as well as for purposes providing automatic color control (ACC) and color killer control.

What is claimed is: 
     
       1. In a receiver for processing a color television signal including chrominance and burst components exhibiting a prescribed mutual timing relationship and alternating in phase on a line by line basis, a chrominance channel for processing said color signal, a plurality of color demodulators for demodulating selected phases of said chrominance component, means for providing line by line switching control of one of said demodulators, and wherein proper operation of said receiver requires that said demodulator be switched in correct timing synchronism with line by line alternation of signals received by said receiver, switching control apparatus comprising: timing means for providing half line frequency timing signals, said timing means being coupled to said switching control means for determining said line by line switching;   means for providing keying signals coincident with said burst component;   sampling means responsive to said timing and keying signals for periodically sampling signals processed by said chrominance channel during alternate line intervals, for deriving an identification signal representative of the timing condition of said burst component; and   means for coupling said identification signal to said timing means for controlling the operation of said timing means such that timing signals corresponding to correct line to line switching are provided.   
     
     
       2. Apparatus according to claim 1, wherein: said chrominance channel includes detector means responsive to said burst component for providing a signal representative of said burst component; and   said sampling means is coupled to said detector means for deriving said identification signal from said burst representative signal.   
     
     
       3. Apparatus according to claim 2, wherein: said timing means comprises a bistable circuit being disabled in response to said identification signal when present.   
     
     
       4. Apparatus according to claim 2, wherein: said timing and keying signals exhibit a mutual timing relationship such that said identification signal is produced when said sampling means samples a predetermined characteristic of said burst representative signal.   
     
     
       5. Apparatus according to claim 4, wherein: said sampled characteristic corresponds to the magnitude of said burst representative signal.   
     
     
       6. Apparatus according to claim 4, wherein: said sampling means includes charge storage filter means; and   said sampling means is rendered operable between relatively low and high impedance states for alternately coupling said burst representative signal to said filter means and for uncoupling said last named signal from said filter means so as to store information representative of said characteristic at said filter means.   
     
     
       7. Apparatus according to claim 6, wherein: said filter means exhibits a time constant determinative of the response time of said sampling means.   
     
     
       8. Apparatus according to claim 7, wherein said sampling means comprises: controllable conduction means defining a signal path for said burst representative signal to said filter means; and   switching means coupled to said controllable conduction means and responsive to said timing and keying signals, for rendering said controllable conduction means conductive during each sampling interval for coupling said burst representative signal to said filter means, and substantially non-conductive during each non-sampling interval of each operating cycle.   
     
     
       9. Apparatus according to claim 1, wherein said receiver further comprises: means coupled to said chrominance channel for automatically controlling the magnitude of signals processed by said chrominance channel in response to received signals of varying intensity, said controlling means being responsive to output signals from said sampling means.   
     
     
       10. Apparatus according to claim 9, wherein said controlling means comprises: first means for varying the signal gain of said chrominance channel inversely relative to the level of said received color signals; and   second means for inhibiting said chrominance channel in the presence of received color signals of a magnitude below a given level.